<!DOCTYPE html>
<html>
<head>
  <style>
    .nav { display: table }
    .nav:after { content: " " }
  </style>
</head>
<body>
  <div style="font-size: 1rem"></div>
  <div class="nav">
  </div>
  <script>
    document.documentElement.style.fontSize = "10px";
    document.documentElement.offsetHeight;
    document.documentElement.style.fontSize = "15px";
  </script>
</body>
</html>
